1. Field of the Invention
The present invention relates to a system and method for driving multiple loads. More particularly, the present invention relates to a system and method for driving two lamp loads connected in series.
2. Description of Related Art
CCFLs (cold cathode fluorescent lamps) are widely employed in display panels. CCFLs require approximately 1500 Volts (RMS) to strike, and require approximately 800 Volts (RMS) for steady state operation. In displays where two CCFLs are required, a conventional technique is to couple the lamps in parallel with the secondary side of step-up transformer. In multiple lamp systems, the conventional technique for driving the lamps is to couple the lamps together in parallel with one another to the transformer. While this ensures voltage control during striking, this topology also requires impedance matching circuitry for the lamps. Also, current control in this topology is difficult since the current conditions of each lamp must be monitored.
Accordingly, it is desirable to couple lamps in series since current control for series-connected lamps is idealized. However, connecting lamps in series requires the transformer to deliver a multiple of striking voltage for each lamp. This, obviously is untenable since most transformers are incapable of providing 3000 Vrms for striking, or are prohibitively expensive. Thus, there is a need to provide a lamp driving system that can drive two lamps coupled in series without straining the transformer to develop double the striking voltage.
Accordingly, the present invention provides a load driving system, comprising a transformer; a first impedance network coupled in series to a second impedance network, said second impedance network being phase-shifted with respect to the first impedance network, the first and second impedance networks coupled in parallel to a power source. A first load is coupled in series to a second load, the first and second loads are coupled in parallel to said first and second impedance networks.
In another embodiment, the present invention provides a circuit, comprising a first impedance network coupled in series to a second impedance network, said second impedance network being phase-shifted with respect to said first impedance network, said first and second impedance networks coupled in parallel to a power source; and a first load coupled in series to a second load, said first and second loads coupled in parallel to said first and second impedance networks.
In the present invention, the phase difference between the first and second impedance networks ensures that the power source deliver significantly less voltage the loads connected in series. Also, in other exemplary embodiments, the resistance difference between the first and second impedances ensures a desired load striking sequence.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to preferred embodiments, the present invention is not intended to be limited to these preferred embodiments. Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein: